فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri - Paris Observatory

A bump appearing in the plot of stellar → opacity versus
temperature. The ionization of the heaviest → chemical elements,
especially → iron, which is the most
abundant heavy metal, produces a large number of weak spectral
→ absorption lines. These lines
dominate the stellar opacity in the temperature range 105-106 K
and furnish two local opacity peaks: a large peak around 2 × 105 K and a
smaller one around 1.5 × 106 K (Rogers & Iglesias, 1992, ApJS 79, 507;
Iglesias et al. 1992, ApJ, 397, 717).

1) General: The state or quality of being opaque.
2) A measure of the absorption of photons on their way from the stellar center
to the surface. Opacity depends upon the frequency of the radiation,
the density, the chemical composition, and the thermodynamic state of the gas.
For a given density, the hotter the gas the lower the opacity, since the gas absorbs
less readily, as described by → Kramers' law.
Conversely, the cooler the gas the higher the opacity.
See also → stellar pulsation,
→ kappa mechanism, → valve mechanism,
→ partial ionization zone.

The → opacity
of a gas of given composition, temperature, and density averaged
over the various wavelengths of the radiation being absorbed and scattered.
The radiation is assumed to be in → thermal equilibrium
with the gas, and hence have a → blackbody spectrum.
Since → monochromatic opacity
in stellar plasma has a complex frequency dependence,
the Rosseland mean opacity facilitates the analysis. Denoted
κR, it is defined by:
1/κR = (π/4σT3) ∫(1/kν)
(∂B/∂T)νdν, summed from 0 to ∞, where
σ is the → Stefan-Boltzmann constant, T temperature,
B(T,ν) the → Planck function, and
kν monochromatic opacity
(See Rogers, F.J., Iglesias, C. A. Radiative atomic Rosseland mean opacity
tables, 1992, ApJS 79, 507).

Named after Svein Rosseland (1894-1985), a Norwegian astrophysicist, who obtained
the expression in 1924; → mean; → opacity.